Au embrittlement

Previous Literature on Au Embrittlement. Figures 14(a) through (c) plot the tensile strength, shear strength, and elongation as a function of the Au content in eutectic Sn-Pb alloy (Ref 106-108). Although the test data scatter, it is seen that both the tensile and the shear strength increase initially as the Au content in-... 

The Au concentration in the solder joints was estimated to be approximately 0.1 wt%, based on the following dimensions the Au film on PCB pad is 5 pin., the Au coating on BGA pad is 15 pin., and the solder joint is a column with the same diameter of the PCB pad and the height of 0.03 in. This concentration is much smaller than 3 wt%, the limit concentration based on the previous literature on the Au embrittlement. The 3 wt% limit is based on the brittle fracture of the solder joint. The brittle fracture observed here is a new fracture mode, a delamination at the interface, instead of brittle fracture of the solder. 

Control of the Au Embrittlement. A solder joint attached to Ni-Au finish becomes more and more brittle with time because of the gradual deposition of the AU Nii Sn4 at the solder joint/pad interface. One way to restore the reliability of the solder joint is to reflow it every 5 years or so, to dissolve the brittle AU Nii Sn4 back to the solder matrix. This solution, however, may not be practical. 

A. Syed, Reliability of Au Embrittlement of Ixad-Free Solders for BGA Applications, International Symposium on Advanced Packaging Materials, 2001, p 143-147... 

Several solderable and protective finishes along with their thickness as specified by MIL-STD 1276D are listed in Table 5 [5,40]. One of the most commonly used solderable-plus-protective layer combinations is Ni and Au. The likelihood of Au embrittling Pb n solder joints is reduced by a hot-solder dipping operation that replaces the Au with a Pb-Sn finish [41,42]. Copper is a frequently used solderable coating it is often coated with a Pb n or Sn protective finish (electroplated, plated and fused, or hot-solder dipped). Other protective layers include... 

Brazing of CoCr-alloys can be performed either in a hydrogen atmosphere or in vacuum. As filler metals Ni- or Co-base alloys or Au-Pd-alloys can be used. In order to achieve a better wetting by the filler metals an electroplating or flashing with Ni is performed. Copper filler metals should be avoided because of the danger of an embrittlement of the seam. Table lb.8 shows the compositions of a typical Co-base filler metal. 

It was found In this study that the addition of Pd to the Au-NI system Increases the high temperature capability of the alloys at the cost of strength, ductility.and more significantly, brazablllty. The low strength of 60Pd-40Nl, with an Increased modulus of elasticity. Indicates the embrittlement of this alloy. Based on this study. It seems that Au-Pd-Nl system Is limited to the capacity obtainable from Au-Ni system with up to 10% of Pd addition. 

Thus, if carbon is adsorbed to the interface, this will reduce /Au/sapphire- If it also reduces Wad, then it must be adsorbing to either the Au surface or the sapphire surface, or both, with an even larger reduction in yau and/or /sapphire- This situation is analogous to the phenomena of grain-boundary embrittlement described in Chapter 7. 

Solder Bath Contamination. Solder bath contaminant levels should be regularly monitored and limited to levels found in lPC-S-815. Many metals found on component terminations will dissolve into molten eutectic Sn-Pb solder. High Cu concentration is a relatively common occurrence that is associated with a rough solder surface and causes poor solder-ability. High Au concentrations can embrittle solder joints (see Sec.57.5.1.3 for a discussion of this phenomenon). 

Not all metal.s and alloys can be persuaded to fracture in an intergranular manner by impact at low temperature. However, a whole group of metals and alloys will fracture in that mode if they are first embrittled by charging with hydrogen. Some will then fracture intergranularly by impact, but others require to be fractured by a slow tensile pull. This group includes the au.stenitic stainless steels [4], nickel [12], the nickel-base superalloys [14] and nickel aluminum alloys [45]. 

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